The present invention relates to optical instruments which process wavelengths of electromagnetic radiation to produce an interferogram. More particularly, the present invention relates to instruments (e.g., Fourier transform spectrometers) which produce interferograms of a scene, which instruments include an optical system which both splits the incoming wavelengths and spectrally disperses them to produce two sets of spectrally dispersed beams. The dispersion is achieved by a matched pair of gratings or similar system. The instrument is useful in analyzing individual chemical species in absorption or emission spectroscopy where there is a need to image a time and spatially varying scene. This could be, for example, imaging an emission plume for a jet or rocket engine or a smoke-stack.
Imaging spectrometers are, broadly speaking, optical instruments which process the electromagnetic radiation from a source into its fundamental components. For instance, an interferometer divides light from a source and interfers it to produce a fringe pattern of interfering light (i.e., an interferogram). The interference pattern can be captured on film or by, for instance, a semi-conductor array detector (e.g., a charged coupled device (CCD)).
There are numerous optical designs. The basic form of the Sagnac (or common path) interferometer is illustrated in FIG. 1. It is also illustrated in U.S. Pat. No. 4,976,542 to Smith. Other designs include the Mach-Zender interferometer, the Michelson interferometer and Twyman-Green interferometer (See W. L. Wolfe, Introduction to Imaging Spectrometers, SPIE Optical Engineering Press, pp. 60-64, 1997), the Fabry-Perot interferometer (see Wolfe, p. 70-73), the Lloyd""s mirror interferometer (see the Smith patent) and, a variation of the common path interferometer (Sagnac) sometimes referred to as the Barnes interferometer (see T. S. Turner Jr., et al., A Ruggedized Portable Fourier Transform Spectrometer for Hyperspectral Imaging Applications, SPIE Vol. 2585 pp 222-232.) There are also dispersive spectrometers such as prism spectrometers and grating spectrometers. (See Wolfe pp. 50-52 and 55-57).
In a non-imaging Fourier transform spectrometer the point source of radiation is split into two virtual points a fixed distance apart to yield a fringe pattern at the detector. If one wants to attain a fine spectral resolution, the distance between the two virtual points should be large; for a course spectral resolution, it should be short. This distance may be controlled by shifting one of the mirrors (typically referred to as lateral shear) of, for instance, the common path interferometer. With this arrangement, a wide spectral range measurement loses resolution, while a high resolution measurement reduces the effective spectral range. In an imaging spectrometer, the point source is replaced with a slit giving the instrument the capability of one-dimensional imaging in the direction perpendicular to the shear.
In a conventional Fourier transform spectrometer, the interferogram records all spectral frequencies from the zero wavenumber to the upper spectral limit of the detector, even though the detector may not be able to sense this entire range. For a system utilizing a silicon detector that responds to wavenumbers from 10,000 to 25,000 cmxe2x88x921 (400 to 1000 nm), almost half of the information content in the interferogram is from frequencies that the detector cannot sense. The upper spectral limit of the interferogram (referred to as Nyquist limit of the detector) is determined by the ability of the detector to sample the interferogram properly.
Shear, both lateral and angular, is discussed in Turner, Jr. et al. (supra). For the Sagnac, translation of either mirror in the plane of FIG. 1 produces lateral shear. Mirror tilt about on axis perpendicular to the drawing plane also produces lateral shear. Conversely, in the Barnes interferometer only angular shear is possible and is produced only by mirror tilt. See FIGS. 2 and 3 of Turner, Jr., et al.
U.S. Pat. No. 4,976,542 to W. H. Smith discloses a Fourier transform spectrometer which incorporates the common path (or Sagnac) interferometer and in which a charge-coupled device (CCD) is placed in the image plane instead of film. The CCD has pixels aligned along two dimensions to provide both spectral resolution and spatial resolution. The CCD is characterized by greater dynamic range, lower pixel response variation, and is photon nose limited, all of which enhances its use as a detector for a spectrometer. See also Digital Array Scanned Interferometers for Astronomy, W. H. Smith, et al., Experimental Astronomy 1: 389-405, 1991. In these devices, the interferometer introduces a later shear in one direction and a two dimensional camera is aligned so a row of pixels is parallel to this geometric plane. In the perpendicular direction, a set of cylindrical lenses is used to provide an imaging capability along the columns of pixels. A row plot from the detector is an interferogram similar to the interferogram collected in a temporally modulated Michelson interferometer.
In a paper published in 1985, T. Okamoto et al. describe a method for optically improving the resolving power of the photodiode array of a Fourier transform spectrometer by modulating the spatial frequency of the interferogram with a dispersing element. With the use of a dispersing element, particularly an optical parallel, the distance between the two virtual sources varies with the wavenumber (the inverse of wavelength) of the source. Thus, as illustrated in FIG. 2 of this reference, by placing their optical parallel into the optical path of a common path interferometer, the distance between the virtual source becomes a function of the wavenumber (i.e., the optical parallel refracts the blue beam more than the red beam, yielding a wide distance between S1blue and S2blue and a narrower distance between S1red and S2red). The authors claim that use of the optical parallel greatly enhances the resolution. In principle, the spectrometer can be designed to examine any wavelength band of interest by careful choice of the type of dispersive glass utilized and the thickness of the glass. See xe2x80x9cOptical Method for Resolution Enhancement in Photodiode Array Fourier Transform Spectrocopy,xe2x80x9d T. Okamoto et al, Applied Optics Vol. 24, No. 23, pp 4221-4225, Dec. 1, 1985.
The approach of Okamoto et al. has a number of drawbacks. First, because of the use of the dispersive block, the system no longer operates with constant wavenumber increments. This is in contrast with conventional Fourier transform spectrometers, which are constant wavenumber devices and are inherently spectrally calibrated. Thus, with Okamoto et al., blue wavelengths have a much smaller spectral resolution than red wavelengths, and the spectral calibration of the instrument becomes a major issue. Another drawback is that the spectral dispersion, while it enhances spectral resolution, adversely affects spatial resolution. Thus, the dispersive element would greatly increase the complexity of an imaging Okamoto et al. spectrometer. Another disadvantage of this technique is that its dependence on a dispersive material restricts its use to wavelengths that can be effectively transmitted through a dispersive element. Finally, the limited glass types that are available restrict the range of spectral enhancements available. While it is theoretically possible to use any dispersive glass and increase the size of the block to achieve the desired spectral enhancement, in practice the size of the block may become so large that the instrument is no longer practical. Also, since the enhancement depends on the glass type and size, the instrument designer has a limited number of parameters to use to optimize the spectrometer design and may not be able to arbitrarily set the lower and upper limits of the spectral region of interest.
In xe2x80x9cSpatial Heterdoyne Spectrocopy: A Novel Interferometric Technique for the FUV,xe2x80x9d J. Harlander et al., SPIE Vol. 1344, pp. 120-131 (1990), the authors describe an improved interference spectrometer which has no moving parts, can be field widened, and can be built in an all reflection configuration for UV applications, particularly FUV applications. Harlander et al. are addressing a different problem from that addressed in Okamoto et al. and approach their solution in a different manner (e.g., the use of angular shear instead of the lateral shear required by Okamato et al.). The basic concept (illustrated in FIG. 1 of this reference) is based on a Michelson type interferometer in which the return mirrors are replaced by diffractive gratings. These gratings, which disperse the radiation, produce Fizeau fringes (i.e., interferograms) which are recorded by a detector positioned in the image plane. The Fourier transform of the fringe pattern recovers the spectrum. An all reflection version of the foregoing utilizes a collimeter, a diffraction grating and two mirrors. Light from the collimeter is split into two beams by the first half of the diffraction grating, which travel in different directions until they are recombined by second half of the same grating and focused onto the detector by a mirror. This is illustrated in FIG. 2 of this reference. See also, xe2x80x9cSpatial Hetrodyne Spectroscopy for the Exploration of Diffuse Interstellar Emission Lines at For-Ultraviolet Wavelengths,xe2x80x9d J. Harlander et al., The Astrophysical Journal, 396: 730-740, Sep. 10, 1992, and U.S. Pat. No. 5,059,027 to Roesler et al. All the designs suggested/disclosed require the use of collimated light and angular shear.
There are a number of drawbacks/limitations associated with the designs suggested/disclosed in the above referenced Harlander et al. publications and Roesler patent (collectively xe2x80x9cHarlander et al.xe2x80x9d). First of all, Harlander et al. do not disclose the concept of imaging a spatially varying scene. Their invention is discussed in the context of imaging a star or some other type of point source. They implicitly assume that the light coming into their optical system is homogenous and report a single spectra. In many cases this may not be true, and proper measurement of the scene would require spectra for each spatial element in the scene. Secondly, all of the Harlander et al. designs require collimating the input beam. Such designs are inherently more complicated than designs which do not require collimated light. Third, the Michelson design on which their designs are based is inherently less mechanically stable than the common path design, since the interferometer is not self-compensating for motions in the elements of the interferometer. It is also not clear if the concept of Harlander et al. is applicable to instruments which utilize lateral shear, as opposed to angular shear. Fourth, although not explicitly stated, all the designs of Harlander et al. require a re-imaging lens to image the virtual sources at infinity. Finally, Harlander et al. require a complex method for separating wavelengths below the central wavelength from those above the central wavelength. That is, a detected fringe pattern could have two different interpretations, it could be from a source below the central wavelength or above. Harlander, et al. discusses methods for determining the true wavelength.
It is an object of the present invention to provide Fourier transform spectrometer that extends spectral imaging to a level where molecular absorption of emission line features can determined for a one-dimensional image.
It is another object of the present invention to provide a Fourier transform spectrometer wherein the resulting image has one dimension of spatial information and for each spatial element a wavenumber resolution spectra, and wherein a two-dimensional image can be created by scanning the field of view over the scene by, for instance, using a rotating mirror or utilizing the forward motion of the spectrometer (e.g., where it is mounted in an airplane).
It is another object of the present invention to provide a Fourier transformer spectrometer in which both the maximum spectral (Nyquist) frequency and the minimum frequency of the interferogram can be tuned to match the spectral sensitivity of the detector or the spectral region of interest without effecting the spatial resolution.
In addition, it is an object of the present invention to tailor the upper and lower cutoff frequencies to the band of interest, to provide extremely high resolutions over limited regions.
It is a further object of the present invention to provide a Fourier transformer spectrometer with the ability to tailor spectrally the bins in which the shortest and longest wavelengths reside, to allow the application of the Nyquist to only the band of interest.
It is also a further object of the present invention to better utilize the information content in the interferogram, by shifting the zero spectral frequency to a spectral frequency that the system can sense.
It is another object of the present invention to provide an interferometer which includes a set of dispersive elements to achieve the spectral resolution set forth above, while retaining spatial resolution and being unaffected by bandwidth.
It is yet another object of the present invention to achieve the foregoing in an instrument (e.g., a Fourier transform spectrometer) which is inherently spectrally calibrated.
It is yet still another object of the present invention to avoid the use of a dispersive element which adversely effects spatial performance.
It is yet a further object of the present invention to provide for an instrument that does not require collimated light.
It is yet a further object of the present invention to provide for a system that does not require a re-imaging lens.
These and other objects will be apparent from the description which follows.
This invention relates to optical instruments having, inter alia, optics to process wavelengths of electromagnetic radiation to produce an interferogram. The instruments include at least one optical path and optical elements positioned along this path for splitting the wavelengths and spectrally dispersing them to produce first and second sets of spectrally dispersed beams which interfere with each other to produce a plurality of different fringes of different wavelengths. In one group of embodiments, the optics for dispersing the wavelengths includes a matched pair of gratings. The gratings may be reflective or they may be transmissive. The gratings, where transmissive, may: take the form of acousto-optical elements; or may be on parallel surfaces of a single optical element. The optics also includes a beam splitter, positioned along the optical path for splitting the optical path, and first and second mirrors. The gratings may be positioned in a variety of locations along the optical path: between the first and second mirrors; between the beam splitter and one of the first and second mirrors; between the beam splitter and the first mirror and between the beam splitter and the second mirror; or between the beam splitter and one of the first and the second mirrors, and between the first and said second mirrors. Where acousto-optical elements are used, the instruments also include apparatus for adjusting the frequency of the sound used to drive the acousto-optical elements to produce a variable amount of spectral dispersion and thereby adjust the spectral performance of the instrument.
In another group of embodiments, the splitting and dispensing of the wavelengths is accomplished only by the matched pair of gratings. Again, the gratings may transmissive, including AO elements, or reflective. Where transmissive, the gratings are aligned along the optical path in parallel planes, to both split and disperse the wavelengths. The optical system in this case also includes a pair of mirrors positioned between the gratings and on opposite sides of the optical path to fold the split and dispersed wavelengths back toward the optical path. A block is also positioned in the optical path between the gratings to block any undiffracted radiation from continuing along the optical path.
In a third embodiment, the optics for dispersing the wavelengths includes a mirror having a plurality of reflecting surfaces, wherein each of the surfaces reflects a specific wavelength or range of wavelengths within a preselected range of different wavelengths. The mirror has a front surface and the plurality of reflecting surfaces includes the front surface and a series of surfaces parallel to but below the front surface. The plurality of reflecting surfaces is obtained by a series of coatings, each of which reflects specific wavelengths or ranges of wavelengths. In addition, a second mirror having a plurality of reflective surfaces could also be utilized. Thus, the spread between the xe2x80x9credxe2x80x9d and xe2x80x9cbluexe2x80x9d can be spread between two mirrors instead of being just on one.
Finally, the optical system may include a beam splitter for dividing the optical path, a mirror positioned along the optical path, and a grating positioned along the optical path, wherein the mirror is rotatable about an axis perpendicular to the plane of the optical path to produce angular shear.
The instruments can all include a detector for detecting the interferogram and means for processing the detected interferogram to produce spectral information. The instruments can further include a slit positioned along the optical path, a detector for detecting the interferogram positioned along the optical path, and optics for focusing the slit on the detector.
The invention also includes the method of spectrally dispersing the wavelengths to produce first and second sets of spectrally dispersed beams which interfere with each other to produce a plurality of different fringes of different wavelengths with the described instrumentation.